1. Field of the Invention
The present invention relates to an image forming apparatus that is adapted to print image information containing tampering-detection information, and to a tampering-detection apparatus that verifies whether image formation information has been falsified.
2. Description of the Related Art
A variety of conventional art systems are available which protect document data from falsification or tampering. For example, a digest of a document to be protected against falsification is computed using a hash function at a transmitting end. The document is then sent together with the digest to a receiving end. A digest of the document received is produced using a hash function at the receiving end. Then, the digest produced at the receiving end is checked against the digest received from the transmitting end to detect whether any change has been made to the document. However, this method is difficult to apply to documents printed on paper such as securities, checks, and circulars. When a digest is produced based on the data read through an input device such as a scanner, if a printed material has been stained or soiled, the digest may well be different from that added to the printed matter originally. This leads to erroneous detection of falsification.
Japanese Patent Application Laid-Open No. 2001-309157 has proposed one such system that protects printed materials from falsification. With this system, an input device such as a scanner reads information from a paper document that should be protected. Then, a characteristic part of the thus read image, for example, a region including character strings is extracted from the image to produce characteristic information on the sampled region, e.g., the density of black pixels in the region. Then, the information containing the characteristic is printed. Likewise, characteristic information on the document is produced at the receiving end. The characteristic information added to the document received is compared with the characteristic information produced. If a difference between the two items of characteristic information is greater than a predetermined value, then it is determined that the document has been falsified.
With the aforementioned conventional art, if an original document is not accompanied by characteristic information, then the falsification cannot be detected. Thus, the original information should be preserved in a safe place until a document (referred to as notification document) having characteristic information is produced. Such data should be discarded once the notification document has been produced. When notification information is first converted into an electronic version and then the electronic version is transmitted, erroneous detection of falsification may occur depending on the parameters of a printer (e.g., resolution) on the receiving end. Moreover, in order to produce a notification document, it is necessary to scan the entire surface of the original document. This presents a problem of working efficiency.